The present invention relates in general to voltage level shifting circuits and, more particularly, to bi-directional voltage level shifting circuits.
Recent advances in the field of semiconductor integrated circuits have brought about higher levels of integration. Semiconductor manufacturing process advancements are driving the corresponding geometric dimensions for semiconductor devices to decreasingly smaller values. As device dimensions shrink, the number of devices per unit area of semiconductor die grows. Given higher device densities within semiconductor die, a greater opportunity exists that devices, which must interface to one another, operate at incompatible drive levels.
An example of incompatible drive levels can be found in the field of digital cellular telephones where a micro-controller communicates with a smart card reader. The smart card reader interrogates the digital cellular mobile subscriber""s smart card and must then relay subscriber authentication data to the micro-controller, which in turn authenticates the mobile user and returns confirmation data to the smart card. Such micro-controllers typically operate in the range of 2.7 to 6.0 volts and such smart card readers operate at approximately either 3 or 5 volts. As the two devices communicate, a series of data bits represented by logic values xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d are exchanged. A logic xe2x80x9c0xe2x80x9d is represented by both the smart card reader and the micro-controller as 0 volts. A logic xe2x80x9c1xe2x80x9d, however, is represented by 6 volts, for example, by the micro-controller and 3 volts, for example, by the smart card reader. The logic xe2x80x9c1xe2x80x9d must be translated by a voltage level shifting device which will translate a 6 volt logic xe2x80x9c1xe2x80x9d signal to a 3 volt logic xe2x80x9c1xe2x80x9d signal for the case when the micro-controller is attempting to communicate a logic xe2x80x9c1xe2x80x9d to the smart card reader. The voltage level shifting device must also be capable of translating data transmission in the opposite direction, or bi-directionally, such as is the case when the smart card reader is attempting to communicate to the micro-controller.
FIG. 1 displays prior art voltage leveling circuit 12 as required to provide adequate voltage level translation for network 10. Prior art voltage level shifting circuit 12 is shown which translates digital voltage levels between communication devices connected to terminal D1 and terminal D2 Communications device connected to terminal D1 is operating from supply potential Vdd1 and communication device connected to terminal D2 is operating from supply potential Vdd2 where Vdd1 and Vdd2 are at different potential levels. Prior art circuit 12 is employed to translate incompatible data signals exchanged between the two communications devices. Many prior art voltage level shifting devices typically employ a single translating transistor 16. Pass transistor 16 is typically an NMOS transistor whose symmetrical structure enables bi-directional voltage level shifting. A complex bias circuit 14 is connected to the gate of pass transistor 16 and also to the two operating voltage supplies Vdd1 and Vdd2.
To enable the prior art voltage level shifting circuit 12 to correctly translate voltage levels between communications device connected to terminal D1 and communications device connected to terminal D2, bias circuit 14 must perform the following functions. Bias circuit 14 must determine the lesser of the two operating voltages, Vdd1 or Vdd2. Once the minimum voltage level is known, the minimum voltage level must be used to bias the gate of pass transistor 16. The minimum gate voltage is required to bias the gate of pass transistor 16 so as to enable the data transmitted on terminals D1 and D2 to properly bias pass transistor 16 into its on and off state. These functional requirements imposed on bias circuit 14 render the prior art voltage leveling circuit 12 as complex and costly. In addition, the prior art voltage leveling circuit 12 does not protect against electrostatic discharge (ESD) destruction of the gate oxide of pass transistor 16 nor does it provide a method to enable or disable data transfer between terminals D1 and D2.
Hence, a need exists for a voltage level shifting device which does not require complex biasing circuitry, provides for adequate ESD protection and allows for an enabling or disabling of the voltage leveling function.